refactor: Move to zephyr/ include paths.

* Zephyr moved to properly namespaced headers, so major "rip
  the bandaid" commit to move us to those everywhere.

app/src/usb.c

@@ -4,11 +4,11 @@
  * SPDX-License-Identifier: MIT
  */
 
-#include <device.h>
-#include <init.h>
+#include <zephyr/device.h>
+#include <zephyr/init.h>
 
-#include <usb/usb_device.h>
-#include <usb/class/usb_hid.h>
+#include <zephyr/usb/usb_device.h>
+#include <zephyr/usb/class/usb_hid.h>
 
 #include <zmk/hid.h>
 #include <zmk/keymap.h>